In atherosclerosis high density lipoproteins (HDL) are a key target for therapeutic intervention. This proposal will examine how apolipoprotein (apo) AI promotes efficient transfer of excess cholesterol (C) from peripheral cells to plasma HDL. Using mouse models of atherosclerosis, we will test the hypothesis that specificity for apoAI in mediating macrophage (Mphi) C efflux is a function of its ability to dissociate from spherical HDL. Dissociation gives rise to a stable, lipid-poor apoAI that can interact with cellular receptors to mediate energy-dependent C efflux. The first specific aim will examine the in vitro specificity of energy-dependent transport of C from cholesteryl ester loaded Mphi. Multiple exchangeable lipid-free and lipid-poor apoproteins including apoAI, apoAII, apoAIV, and apoE will be compared. The second aim will identify the form(s) of apoAI present in vivo in the intima of an atherosclerotic lesion using immunochemical and physicochemical analyses. The third aim will identify the in vivo mechanism(s) for formation of lipid-poor apoAI and will examine the roles of scavenger receptor-type B1, lipoprotein lipase, hepatic lipase, phospholipid transfer protein and apoAII. The fourth aim will test the hypothesis that the in vivo specificity requirement for apoAI can be bypassed by providing an alternate source of lipid-poor apoAI within lesions. Transgenic mice expressing Mphi-specific apoAI will be produced and will serve as donors in bone marrow reconstitution studies of atherosclerosis-prone mice. This hypothesis-driven proposal will: 1) provide a precise understanding of the lipoprotein composition of interstitial fluids; 2) identify in vivo mechanisms to explain the unique capacity of apoAI to remove C from lesions; and 3) use permanent gene transfer to alter the course of atherosclerotic vascular disease.